The phrase "woody and herbaceous plant material," as used herein, broadly includes biomass, such as wood, logs, slabs, chips, and corn cobs. This term is also intended to include processed cellulosic materials such as pulp, paper board, paper, bagasse, rope, and other biomass of plant origin. According to the present invention the woody and herbaceous materials are converted to charcoal by pyrolytic conversion.
The term "inert" in the context of the present invention means that such compound, composition or material does not react with wood, or its by-products of pyrolysis, at temperatures and pressures attained within the reaction container in practice of the present invention.
A conventional art of charcoal manufacture typically involves an eight day batch cycle of loading, heating the biomass feedstock, cooling, and unloading the charcoal. The yield of a good quality charcoal (on an oven-dry, weight basis) from a conventional process is typically 25%, and rarely exceeds 35%. A good quality charcoal will have a volatile matter content of 35% or less.
Grimmett, U.S. Pat. No. 3,929,585, discloses the heating of wood in a fluidized bed to the temperature range of 138.degree.-538.degree. C. However, charcoal yields of only about 20% (on a dry weight basis) are obtained. Baker, et al., Energy Progress, Vol. 3 (No. 4) pages 226-228, Dec. 19, 1983, also disclose experiments wherein wood is heated in a-fluidized bed. Carbon conversion yields resulting from heating the wood in the fluidized bed in absence of a catalyst are reported in the range of 9% (on a carbon basis at one atmosphere) to 21-27% (on a carbon basis at ten atmospheres). However because charcoal is almost pure carbon whereas the wood feedstock is only about 50% carbon, the wood to char yields obtained by Baker, et al. are only about 50% of the stated values, when expressed on a dry weight basis. Thus, Baker, et al. obtained wood char yields ranging from about 4.5% to 10.5-13.5% on a dry weight basis. These values are in accord with the low yield reported by Grimmett in a fluidized bed.
Mok and Antal, Thermochmica Acta, 68 (1983) 165-186, disclose pyrolysis of wood products at various temperatures and pressures. The highest char yield reported in the paper is about 22%. Mok et al. use an experimental device whereby gas is flowed over the biomass during the pyrolysis reaction, which resembles a fluidized bed condition. The Mok et al.
paper shows that variation of pressure alone does not significantly influence the charcoal yield.
In Atkinson, U.S. Pat. No. 1,538,505, a distillation process is disclosed whereby the carbonaceous material (corn cobs, woodchips, etc.) is first extracted with water and steam. The remaining solid residue is then carbonized in a retort. The reactive gases from pyrolysis are diluted and carried away from the solid substrate in the retort, and there is a low yield of charcoal.
Accordingly, it is an object of the present invention to provide an economical process for efficiently converting woody and herbaceous plant material to solid charcoal having a heat value of about 13000 BTU/lb. on a moisture and ash free basis.
According to the present invention, typical batch turn around time is less than two hours, and the yield of charcoal typically ranges from about 35% to 50% The charcoal made in accordance with the present invention typically has a volatile matter content of less than about 25%, and a heat value of about 13000 BTU/lb.
It is a further object of the present invention to provide an improved process for converting woody and herbaceous plant material.
These and other objects and advantages to the present invention will be readily apparent upon reference to the drawing and the following description.